Package-on-package (“PoP”) technology is widely used for electronics to provide increased functionality and further miniaturization typically demanded for consumer devices.
One major type of PoP technology uses a partial molded bottom package coupled to an upper package by using solder balls or bumps. One disadvantage of this design is the difficulty of controlling warpage of the bottom package. Another disadvantage of this design is that a relatively large solder ball is needed to achieve higher ball stand off, which consumes more surface area (i.e., x-y area). Thus, this design has a larger distance between conductive leads (the distance known as “pitch”), and this is not easily amenable to a fine pitch package—i.e., a package with relatively small distances between conductive leads.
Another major type of PoP technology uses a fully molded bottom package, which evolved from the partial molded package. The fully molded bottom package has less warpage and uses through-mold via (“TMV”) connections. For this design, the through-mold via has a solder bump or ball at the bottom of the via of the bottom package. The upper package is connected by another solder ball that contacts the solder ball of bottom package.
A disadvantage of the fully molded package is the difficulty of obtaining a fine pitch for leads using through-mold via connections. To form a TMV, a cavity is formed in the molded epoxy of the bottom package using a laser drilling process. Unfortunately, laser drilling results in the via having a conical shape rather than a cylindrical shape. The conical via shape means that the pitch of the interconnects increases with the thickness of the epoxy mold cap. Moreover, for tight pitch designs, during processing the mold separation between vias may be removed, resulting in shorted solder balls. This may result in higher manufacturing costs for deeper through-mold via designs.
To avoid shorted (i.e., bridged) solder balls, a design using copper pillars or copper posts has been proposed. For that approach, the via does not extend all the way through the mold. Instead, a copper pillar or copper post is built up on the bottom substrate. This reduces the depth of the conically-shaped via in the mold because the via only needs to reach the top of the copper pillar or post. This helps to reduce the via pitch—i.e., the distance between vias. A disadvantage of the copper pillar or post design is that the cost of additive copper plating in very expensive. Moreover, the current state of the art for the industry would be a maximum height of 100 to 150 microns for the copper post.